Tower concentration solar power plants are known in which the solar radiation is reflected by a series of mirrors, called heliostats, toward a central solar receiver that converts the solar radiation energy into a hot fluid that can be next used to produce electricity.
Heliostats are provided with two rotation mechanisms making it possible to track the sun and always return the solar flux toward a given point, irrespective of the time of the day and current season.
The solar receiver is installed at the apex of a tower in order to receive the solar radiation from all of the heliostats without a given heliostat hindering the flux reflection of a neighboring heliostat.
The hot flux generated in the solar receiver may be high-pressure and high-temperature steam generated from feed water. The steam can then be used directly in a steam turbine driving an electricity generator.
A solar power plant of this type has a relatively high installation cost, and all solutions are considered in order to reduce that cost.
Among the solutions that are considered, power increase is an important factor due to scale effect.
Another solution is to increase the cycle output: when the output increases, the number of heliostats can be reduced for a same output power, and therefore the height of the tower is reduced, and so forth. The output of a steam cycle increases when the pressure of the cycle and the steam temperature increase.
The powers considered today greatly exceed 100 MW, with a steam pressure of nearly 200 bars and a steam temperature of nearly 600° C. With these performance levels, the solar receiver can comprise only one evaporator and one superheater: re-superheating can therefore advantageously be avoided, and thus is associated costs as well.
The solar receiver is made up of tube walls receiving the solar fluxes from the heliostats and transmitting heat toward the water and/or steam contained in the tubes. Two distinct exchangers form the solar receiver: the evaporator, converting the feed water into saturated steam, and the superheater, increasing the saturated steam temperature until the desired value is reached.
Between the two exchangers is the drum, which is a very important reservoir of a boiler. The drum supplies the evaporator with its water, via a pumping system. Indeed, at these high pressures, natural circulation is no longer possible, the difference in density between the water and the steam being too low. It is additionally necessary to ensure a sufficient circulation rate to avoid dirtying of the evaporator by the salts contained in the water, but also to avoid drying of the inner surface of the tube.
The drying of the inner surface of the tube (dry-out) is also an important data of an evaporator: if it occurs, the exchange coefficient between the water or the water-steam mixture and the tubes decreases abruptly, the tube is no longer cooled down and can no longer discharge the heat received in the form of radiation.
Overheating occurs, which may lead to the destruction of the tube. The effect also exists in certain evaporation regimes called DNB (Departure from nucleate boiling).
The drum receives the feed water at a flow rate equal to the flow rate of the steam produced by the evaporator and that is exported from the drum. The incoming feed water is mixed with the saturated water contained in the drum.
The drum separates the water from the steam coming from the evaporator: the water recirculates toward the evaporator and the steam is exported, after drying in the equipment provided to that end inside the drum.
In the configuration described above, the water sent toward the evaporator via the circulation system is at the saturation temperature. Evaluating the maximum admissible critical flows leads to a major difficulty regarding drying (Dry-Out/DNB) due to the very high pressure.
A boiler arrangement with two drums connected to one another, for example superimposed, is found in a certain number of patents, sometimes very old, for the reasons specified in those documents.
For example:                in the steam generator of FR 678,909, to facilitate the water/steam separation in order to export a dry steam. This concern is also encountered in document DE 285 489;        in the double-drum tube boiler of document GB 529,444, to have better circulatory characteristics;        in the heat recovery steam generator of WO 2012/148656 A1, to reduce the diameter and therefore the thickness of the drum, and to increase the operating flexibility (shorter startup time, operating temperature of the evaporator reached more quickly);        in the heat recovery steam generator of WO 2012/129195 A2, also to reduce the diameter and therefore the thickness of the drum, and consequently the temperature gradient through its wall upon startup, which increases the thermal fatigue on the drum and in turn causes wear thereof in the form of cracks. The thickness reduction of the drum also makes it possible to reduce the manufacturing cost. These concerns are also found in document EP 1 526 331 A1.        
Document US 2010/0236239 A1 describes a method and a generator for producing steam for an electric turbine plant using solar radiation. This radiation is directed onto a solar receiver. The solar receiver includes a first section, which has a feed water inlet and is arranged to heat this incoming feed water in order to generate steam by using the directed solar radiation. The feed water flows through a feed water vessel to serve as feed water intake at the inlet of the first section of the receiver. The water is separated from the steam in a steam separation vessel, which is in fluid communication with an outlet of the first section of the receiver. The feed water intake can be selectively preheated by a preheating source other than solar energy, in particular electric preheating, in response to the operating conditions of the system, during daylight hours or at the hourly electricity rate. A forced circulation pump allows the fluid to cross through the preheater. The incident solar flux on the solar receiver is at most in the 130-230 kW/m2 interval in maximum solar mode. Furthermore, the temperature reached by the superheated steam is 540-560° C., at a pressure in the range of 100-140 bars. These operating conditions are too low to cause the dry-out phenomenon of the steam-generating tubes. This document does not teach any solution in the event the steam generator must cope with the difficulties related to very high pressure (from 180 bars to more than 200 bars).
In summary, double-drum steam generators have been known for some time in order to produce a dryer usable steam or to reduce the fatigue of the materials or cost of the system.
In no case is mention made of the problem of drying (Dry-Out/DNB). In all of the scenarios, the boilers covered by these patents work indeed at a much lower pressure and with much lower heat fluxes than in the scenario of the present invention, and therefore without any risk of dry-out.